1. Field of the Invention
The present invention relates to a power supply apparatus and an image forming apparatus and, in particular, to a power supply apparatus that includes a current resonance power supply.
2. Description of the Related Art
FIG. 6A is a diagram illustrating an overview of a circuit of a conventional current resonance power supply apparatus (hereinafter, simply called power supply apparatus). Detailed description of FIG. 6A will be made later. A power supply apparatus as illustrated in FIG. 6A detects overcurrent, based on a current detection voltage that occurs across both terminals of a current detection resistor 109 and is input into an OCP terminal of a power supply control IC 110. In the power supply apparatus illustrated in FIG. 6A, the resistor 109 is connected to a resonance circuit in series. Resonance current flows through the resistor 109. Accordingly, the power loss is large. The larger the current flowing through a load 128 is, the larger the normal rated power of a resistor required to be adopted is. Accordingly, the efficiency of the power supply apparatus is reduced. This reduction causes the need to adopt a resistor having a large normal rated power. Accordingly, the cost increases by that for the resistor. In the case of connecting multiple resistors in parallel or series to configure a current detection resistor 109, a large space is required. Thus, for instance, Japanese Patent No. 3013697 proposes a power supply apparatus as illustrated in FIG. 6B. Detailed description on FIG. 6B will be made later. The power supply apparatus in FIG. 6B converts, into a voltage, current flowing through a current detection capacitor 201 connected to a current resonance capacitor 108 in parallel, and inputs the voltage into an OCP terminal of the power supply control IC 110, thereby detecting overcurrent.
However, when the input AC voltage varies, current flowing through the resistor 109 in the case of the power supply apparatus in FIG. 6A and current flowing through the capacitor 201 in the case of the power supply apparatus in FIG. 6B vary inversely with variation in the input AC voltage. As a result, there is a possibility of performing an overcurrent protection operation with an inconstant value of current flowing through the load 128. To address this point, for instance, International Publication No. WO2011/132275 proposes a power supply apparatus as illustrated in FIG. 7A. FIG. 7A will be described later in detail. The power supply apparatus in FIG. 7A has a configuration of applying a correction voltage in proportion to the input AC voltage. This configuration suppresses variation in current value detected by a current detection circuit due to variation in voltage of a commercial AC power supply. The power supply apparatus illustrated in FIG. 7A can perform a more stable overcurrent protection operation than the power supply apparatuses in the foregoing FIGS. 6A and 6B even if the input AC voltage varies.
However, in the power supply apparatus illustrated in FIG. 7A, a correction voltage (1402) to be applied to the current detection circuit exhibits linear characteristics in proportion to the input voltage, as illustrated in FIG. 7B. In contrast, the output voltage (1401) of the current detection circuit exhibits nonlinear characteristics to variation in voltage of a commercial AC power supply. Accordingly, as illustrated in FIG. 7B, this apparatus exhibits a waveform 1403 similar to the waveform of the output voltage of the OCP terminal of the conventional power supply control IC 110, in comparison with an ideal output voltage of the OCP terminal (broken line) of the power supply control IC. Accordingly, this apparatus cannot exert sufficient correction effects against variation in voltage of a commercial AC power supply.
Recently, in every field, reduction in size and energy of products has been advanced. As a result, there is a tendency that reduction in size and energy of a power supply apparatus has also been advanced. In the case of a current resonance power supply apparatus, increase in current flowing through a load 128 causes a phenomenon that flow-through current occurs in both switching FETs 106 and 107 (hereinafter, simply called an off-resonance phenomenon). There is a possibility that the off-resonance phenomenon causes countercurrent to flow through the body diode of an off-state FET between the switching FETs 106 and 107, thereby breaking the off-state FET. As described above, in the case of performing an overcurrent protection operation with an inconstant value of current flowing through the load 128, it is required to prevent an off-resonance phenomenon from occurring in a condition of an AC voltage input when the maximum current flows through the load 128. This requirement increases the size of a transformer 115 and the current resonance capacitor 108, which are main parts for determining the output voltage. This increase causes a problem against reduction in size of the power supply apparatus.